SpringMVC源码总结（七）mvc:annotation-driven中的HttpMessageConverter

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这一篇文章主要介绍下HttpMessageConverter整个注册过程包含自定义的HttpMessageConverter，然后对一些HttpMessageConverter进行具体介绍。

HttpMessageConverter接口介绍：

public interface HttpMessageConverter {

	/**
	 * Indicates whether the given class can be read by this converter.
	 * @param clazz the class to test for readability
	 * @param mediaType the media type to read, can be {@code null} if not specified.
	 * Typically the value of a {@code Content-Type} header.
	 * @return {@code true} if readable; {@code false} otherwise
	 */
	boolean canRead(Class clazz, MediaType mediaType);

	/**
	 * Indicates whether the given class can be written by this converter.
	 * @param clazz the class to test for writability
	 * @param mediaType the media type to write, can be {@code null} if not specified.
	 * Typically the value of an {@code Accept} header.
	 * @return {@code true} if writable; {@code false} otherwise
	 */
	boolean canWrite(Class clazz, MediaType mediaType);

	/**
	 * Return the list of {@link MediaType} objects supported by this converter.
	 * @return the list of supported media types
	 */
	List getSupportedMediaTypes();

	/**
	 * Read an object of the given type form the given input message, and returns it.
	 * @param clazz the type of object to return. This type must have previously been passed to the
	 * {@link #canRead canRead} method of this interface, which must have returned {@code true}.
	 * @param inputMessage the HTTP input message to read from
	 * @return the converted object
	 * @throws IOException in case of I/O errors
	 * @throws HttpMessageNotReadableException in case of conversion errors
	 */
	T read(Class clazz, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException;

	/**
	 * Write an given object to the given output message.
	 * @param t the object to write to the output message. The type of this object must have previously been
	 * passed to the {@link #canWrite canWrite} method of this interface, which must have returned {@code true}.
	 * @param contentType the content type to use when writing. May be {@code null} to indicate that the
	 * default content type of the converter must be used. If not {@code null}, this media type must have
	 * previously been passed to the {@link #canWrite canWrite} method of this interface, which must have
	 * returned {@code true}.
	 * @param outputMessage the message to write to
	 * @throws IOException in case of I/O errors
	 * @throws HttpMessageNotWritableException in case of conversion errors
	 */
	void write(T t, MediaType contentType, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException;

}

从HttpInputMessage中读取数据： T read(Class clazz, HttpInputMessage inputMessage)，前提clazz能够通过canRead(clazz，mediaType)测试。
向HttpOutputMessage中写入数据：void write(T t, MediaType contentType, HttpOutputMessage outputMessage)，前提能够通过canWrite方法。

简单举例：
如StringHttpMessageConverter，read方法就是根据编码类型将HttpInputMessage中的数据变为字符串。write方法就是根据编码类型将字符串数据写入HttpOutputMessage中。

HttpMessageConverter的使用场景：
它主要是用来转换request的内容到一定的格式，转换输出的内容的到response。
看下自定义的使用方式：

首先还是在对mvc:annotation-driven解析的AnnotationDrivenBeanDefinitionParser中，有这么一个方法：

ManagedList messageConverters = getMessageConverters(element, source, parserContext);

获取所有的HttpMessageConverter，最终设置到RequestMappingHandlerAdapter的private List> messageConverters属性上。看下具体的获取过程：

private ManagedList getMessageConverters(Element element, Object source, ParserContext parserContext) {
		Element convertersElement = DomUtils.getChildElementByTagName(element, "message-converters");
		ManagedList messageConverters = new ManagedList